A. INTRODUCTION

• Sky phenomena and motions were important to most human cultures

• Astronomical time cycles were recognized and studied by many cultures

• Tracking astronomical cycles encouraged development of certain technologies:
  • Systematic, persistent observations
    
  • Multi-generational methods of record-keeping (often no traces left)
    
  • Skilled design of observing "instruments"---e.g. special alignments in buildings
    
  • Basic types of geometry and counting/arithmetic
    

B. GREEK ASTRONOMY (ca. 500 BC - 200 AD)

In earlier (and many later) cultures, cosmologies were mythological or supernatural. They had a strong "projective" tendency: human characteristics, inflated to supernatural proportions, were imposed outwards on the cosmos. Direct, persistent, supernatural control of sky phenomena was assumed.
The Greeks had enormous impact, both because they were remarkably innovative and they left a large, coherent body of written records. They developed the Western versions of: literature, history, philosophy, logic, mathematics, & science. Not bad work. First (recorded) scientific interpretation of astronomy
Flourished 500 BC - 200 AD; but Greek science was rediscovered during the Renaissance & was therefore influential until 1500 AD. ===> a span of 2000 years!
Early discoveries in mathematics (e.g. Pythagorean theorem, irrational numbers, plane geometry) became the basis of their scientific approach. Pythagoras: "all things are numbers."

Extract from Aristarchus' study of the distances to the Moon and Sun

• The spherical shape of the Earth & Moon
  
• The origin of lunar phases
  
• The origin of eclipses
  
• The existence of precession of the equinoxes (Hipparchus).

• The approximate distance to the Moon & Sun (Aristarchus, see extract above)

• The diameter of Earth to an accuracy of about 150 miles (Eratosthenes, ca. 200 BC). Erathosthenes' method uses simple measurement of shadow lengths & the congruence of triangles and is shown in the diagram below. (The different shadow lengths are also, of course, part of the evidence that the Earth is a sphere.)
  

• They treated the Sun, Moon, planets, stars as physical objects, not living beings with supernatural volition and powers.

• Influenced by the philosophical idealism of Plato & Aristotle, they attempted to deduce the character of nature from abstract postulates (like mathematical axioms), with little appeal to empirical tests and with explicit dismissal of experiments.
  • On philosophical grounds, they favored a highly symmetrical (spherical), perfect universe.

  • They attached special, but arbitrary, characteristics to basic building blocks: "earth," "air," "ether," etc. A primitive but nonetheless recognizable version of the modern atomic interpretation.

  • Although a revolutionary improvement over supernatural interpretations, this reliance on deduction (instead of empirical investigations) ultimately misled Greek astronomers.

• Greek models were intended to be consistent with their large and accurate collection of observations of the Sun, Moon, planets, and stars.
  
• The first attempts at serious cosmological models placed Earth at the center of the universe (geocentric) and introduced the notion of "crystalline spheres" concentric with Earth, each carrying a celestial object. Click on thumbnail at right for larger view.

• Aristarchus (ca. 250 BC) proposed a heliocentric (sun-centered) cosmos, based on his realization that Sun was probably larger than Earth, but this was not favored.

• Ultimate model by Ptolemy, ca. 130 AD:
  • The model is geocentric, with the Earth sitting stationary at the center of a spherical universe. The terrestrial region is regarded as corrupted and changeable, but at larger distances from Earth, the universe becomes ideal, perfect, unchanging.

  • Circular (ideal) celestial motions of all objects about Earth

  • Earth does not spin on its axis; rather, the universe revolves about Earth once a day.

  • But in the real solar system, the Earth moves and the planetary motions are not perfectly circular, therefore:
    • Ptolemy had to include a number of complicated geometric features in order to reproduce the observed planetary motions.

    • Viewed from Earth, the planets all appear to undergo occasional "retrograde motion"---a brief, loop-like reversal in their general eastward motion with respect to the stars. This is readily visible in the computer planetarium simulations.

    • To reproduce such motions, Ptolemy's model used "epicycles" (wheels moving on wheels). See the illustration below. The epicycles were purely geometrical constructs, without any presumed physical reality to them.

    

    
    

  • Here is an animation of a Ptolemy-like model.

Ptolemy's work is often treated dismissively because it "got the solar system wrong" and was discarded by the "Copernican Revolution." However, it is important to appreciate how enormous a step forward this was over all the other modes of thinking at the time and, in fact, over any other framework for understanding the universe for the next 1300 years(!)
Science is a cumulative and pan-cultural enterprise. It discards bad ideas which are found to be empirically unsupported but retains useful ones. Many features of the cosmology of the Greeks propagated through to modern science, including these:
• They attempted to incorporate all of the extant reliable data.

• They insisted that theoretical models reproduce the observations.

• They regarded the planets, Moon, and Sun as inanimate, physical objects moving through space without supernatural interference. This was a tremendous break with the interpretations of almost all other cultures of the time.

• Their models were based on mathematics. All later science likewise used mathematics (of an ever-increasing sophistication). The modern view is that although all things may not BE numbers (as the Pythagoreans claimed), all things can be measured by numbers.

• The models emphasized geometrical symmetry. Much later (the 20th century), symmetry was found to be the key to understanding subatomic particles, crystalline solids, DNA molecules, and a wealth of other phenomena.

Ptolemy's model reproduced the angular motions of the planets on the sky reasonably well. Despite flaws, this is a scientific model which makes predictions that can be tested: e.g. concerning the brightnesses of the planets and their distances from the Earth as they move around their complex orbits. Although the Greeks apparently did not test the models this way, later observers like Tycho could easily do so.

C. DARK INTERLUDE AND RENAISSANCE

D. COPERNICUS (d. 1543)

• A fundamental change in perspective: Earth is now merely one among the (six) known planets. It has been "dethroned" from its privileged location at the center of the universe. The Sun becomes the most important object in the solar system.
  • The extent to which conditions on the other planets may closely resemble those on Earth was not known to C. (without telescopes), but there was no evidence then that they were very different. A "multiplicity of worlds" therefore emerges, possibly inhabited.

• C's arguments were based on mainly on: (a) simplicity; and (b) the recognition that motions of the planets in P's model (e.g. retrograde loops) were synchronized with the motion of the Sun, which implied the Sun was the key object.

• C had no conclusive observational evidence of Earth's spin or revolution around Sun.
  • Such evidence became available only much later, with telescopes and other instruments which could measure, for instance, the "aberration of starlight" caused by the Earth's orbital motion around the Sun (Bradley, 1729) or the "coriolis effect" caused by its spin (best demonstrated by the Foucault pendulum--1851).

• C also was forced to assume that the stars were very distant in order that the "parallactic shift" caused by viewing them from different positions in Earth's orbit would be too small to measure.
  • This implied an enormously larger universe than most astronomers were willing to accept at the time.

  • The first measures of the parallactic shifts for nearby stars (about 1 arcsecond) were made by Bessel in 1838, almost 300 years after Copernicus' death. Using these shifts, stellar distances can be calculated by simple trigonometry. The nearest stars are at distances over 200,000 times the radius of the Earth's orbit. Even Copernicus himself would have been flabbergasted by the scale of our star system.

• In P's model, the universe must rotate around the Earth once a day. It therefore cannot be very large. But in C's model, this diurnal motion is caused by the Earth's spin. The universe is stationary. This permits it, in principle, to be infinite in extent.

• In C's model, the planets continuously move in the same direction (counterclockwise around Sun as seen from above Earth's north pole). Planets nearer the Sun move around faster. Retrograde motions are explained as the reflex of the Earth's orbital motion (i.e. the fact that we observe the other planets from a moving platform). For instance, Mars appears to move backwards in our sky as the Earth "catches up to and passes" it in its orbit. See the animation below:
  
• C's system still assumed circular motions for objects and still required epicycles (since planetary orbits aren't pure circles)

• Here is an animation of C's model.

Reading: FMW: 1.2 and 1.4
Optional references: Bertrand Russell, A History of Western Philosophy; Arthur Koestler, The Sleepwalkers; Timothy Ferris, Coming of Age in the Milky Way; J. L. E. Dreyer, A History of Astronomy from Thales to Kepler.

• Early Greek Astronomy (to ca. 300 BC)
  
• Biography of Aristarchus
  
• Essay on Aristarchus
  
• More detailed notes on Greek science from Mike Fowler, UVa PHYS 109.
• Biography of Ptolemy
  
• More information on the Ptolemaic model
  
• More information on the Copernican system
  
• Manuscript copy of Copernicus' De revolutionibus